L-type Ca²⁺ currents and SR Ca²⁺ release flux in muscle fibers from WT and IT/+ mice. (A) Voltage dependence of L-type Ca²⁺ current density. (B) Normalized Ca²⁺ conductance derived from the data shown in A (V_1/2, k, and G_max were −4.38 ± 1.58 mV, 4.66 ± 0.28 mV, and 154 ± 14.6 SF⁻¹ for WT and −4.57 ± 1.41 mV, 5.33 ± 0.20 mV, and 162 ± 15.3 SF⁻¹ for IT/+, respectively). Mean membrane capacitance was 5.06 ± 0.25 nF (WT) and 4.72 ± 0.38 nF (IT/+). (C) Voltage-activated changes in fura-2 fluorescence ratio. All mean values above −20 mV were significantly (P < 0.05) different, in contrast to the parameters V_1/2 and k, which were not significantly different (−18.5 ± 1.29 mV and 5.15 ± 0.24 mV for WT and −21.6 ± 1.24 mV and 5.17 ± 0.30 mV for IT/+, respectively). (D) Normalized peak Ca²⁺ release flux. Values from the different experiments were averaged after normalization to the maximum. V_1/2 and k were not significantly different (−10.0 ± 1.9 mV and 7.54 ± 0.56 mV for WT and −12.2 ± 1.19 mV and 8.54 ± 0.46 mV for IT/+, respectively). The absolute maximal values (assuming internal EGTA concentration to be 40% of the pipette concentration) at +50 mV were 64.6 ± 17.5 and 49.9 ± 9.4 mM/s, respectively. Current and fluorescence signals were simultaneously recorded in isolated interosseus fibers (WT, filled circles and continuous lines, n = 15; IT/+, open circles and dashed lines, n = 14).